K2Ska311

When getting off the accelerator in any gear, the car will begin to slow down immediately, then about a half second later it will lurch forward before beginning to decelerate again. Likewise when getting on the pedal, it will begin to accel, hesitate for a second, and then proceed to accelerate again. This only happens under 3k rpms, it acts normally > 3k. I've had theories that it's the tune, throttle body, and even diff. I was just wondering if anyone has experienced this problem and had a more definitive answer before I start spending money. Any help would be appreciated, thanks in advance.

YoshiFD3S

iTrader: (19)

Sounds like issues with your primaries + in need of a re-tune, perhaps?

slpin

iTrader: (9)

tps

K2Ska311

If it was the tps wouldn't it happen over 3krpms as well?

TpCpLaYa

iTrader: (10)

not necessarily.


I'd double check all your grounds first

DaleClark

iTrader: (55)

Few things it could be.

Do you have a stock ECU? Is your airpump running properly? The stock ECU will be jerky on/off throttle without the airpump running. The ECU is fudging the O2 sensor's output to account for the extra air from the airpump throwing off the O2 sensor's readings.

Next is the O2 sensor itself. The ECU uses its feedback at low load/throttle inputs.

Dale

K2Ska311

Grounds are good as I just did a motor swap and re-grounded everything. (Problem happened both before and after swap) Running PowerFC as well.

DaleClark

iTrader: (55)

With the PowerFC, do you have O2 sensor feedback turned off? Even on a bone stock car with a PFC, I do that - car drives way better without the O2 sensor, and the fuel economy is identical (I tested that out myself) .

Dale

K2Ska311

I'll go check that right now Dale, thanks.

Captain_Panic

iTrader: (15)

Mine does similar things letting off the throttle. I think my Diff mount bushings are part of the problem as I get A LOT of shifter movement in 3rd gear when I get off the throttle. I am getting the Mazda Comp Bushing in a diff to replace to see how that changes the cars reaction. I am also replacing my Air Pump with the PineApple Racing Pulley today. Between these two pieces (and probably a retune shortly there after with a few other Mods) I will see how it plays out.

Oh and I have the PFC with the O2 disabled as well.

K2Ska311

Yeah it's been off it seems =\

DaleClark

iTrader: (55)

Another possibility is a broken powerplant frame, letting the drivetrain settle back and forth. Diff bushings are pretty hardy and don't fail terribly often.

Dale

Captain_Panic

iTrader: (15)

PPF was my first thought on mine too, but it looked great when I attached my Banzai diff brace.

K2Ska311

I've also noticed that the hesitation is significantly worse when the car is warmed up and much less noticeable when it's cold if that helps at all.

arghx

iTrader: (3)

tip-in needs to be tuned, this is unfortunately very normal for the PFC without a lot of time spent making adjustments in the Datalogit. And even then it's hard to get it to be as responsive as stock.

K2Ska311

I certainly hope it's just the tune since I need to have the tune tweaked for the new motor anyway. My fear is that it's not the tune and when I get it re-tuned it won't be accurate due to whatever the problem really is. (injectors, TB, TPS, etc...)

Captain_Panic

iTrader: (15)

That is the information I was looking for and is probably what the OP needs too. My car has only been Dyno tuned, and doing some road based tuning is next on my list once the snow melts here in Chicago.

Thanks for this!

arghx

iTrader: (3)

there are multiple tables that affect the tip-in. There is Accelerate Injector (which is available in the Commander). And then there is INJ vs Accel TPS1 and INJ vs TPS, which are available only in the Datalogit. They all have slightly different effects on the amount of additional fuel delivered as the throttle opens.

The best way to do it is to log "Advance" in the monitor window of the Datalogit software, along with a wideband input (Aux A/D in monitor window). Log nothing else as it will slow your sample rate. Then create graphs of InjFrPr (primary injector pulsewidth), TPS voltage, and wideband AFR, and RPM. Log a full drive cycle (round town and highway low load driving). Look over the logs for a while, and note instances where AFR is lean and TPS voltage is increasing.

Then find a neighborhood or large parking lot. You need to be making slight movements of the pedal while coasting in 1st and 2nd gear. Then proceed with adjusting the settings from there. They key is the relationship between the rate of change of throttle movement (call it dTPS) and the amount of the additional fuel being supplied. I know that sounds... really technical, and it kind of is, but on a practical level it comes down to methodical trial-and-error with a coupe different tables in the PFC.

There may be a few random threads here or in the Power FC section where I've discussed this in some greater detail, as well as my FC3S Power FC guide (which I don't think is up anymore). It's often a time consuming process and can be as difficult as getting the idle right.